refactoring: tidy up library and reuse more logic

[davidhewitt]

This PR tries to re-use more of the library logic into repeatable patterns. I also bumped the minimum Python version to 3.6, made liberal use of f-strings, and changed many error messages to start with lowercase.

@tiran would you be willing to pass a quick review over this? In particular the new RustCommand base class should ensure that optional extensions failing will always print a nice message (thanks to except Exception on line 45 of command.py).

Fixes #105

[tiran]

Excellent work! optional=True now ignores cargo metadata errors:

...
running build_ext
running build_rust
error: failed to download `pyo3 v0.13.1`

Caused by:
  can't make HTTP request in the offline mode
build_rust: optional Rust extension _rust failed
Command '['cargo', 'metadata', '--manifest-path', 'src/rust/Cargo.toml', '--format-version', '1']' returned non-zero exit status 101.
generating cffi module 'build/temp.linux-x86_64-3.9/_padding.c'
creating build/temp.linux-x86_64-3.9
generating cffi module 'build/temp.linux-x86_64-3.9/_openssl.c'
building '_openssl' extension
creating build/temp.linux-x86_64-3.9/build
creating build/temp.linux-x86_64-3.9/build/temp.linux-x86_64-3.9
...

[davidhewitt]

😄 thanks for confirming!